If you want to select a language that is not listed in the steam properties, but that is listed in the game folder "locale".

There is a config file, config.js, in the game folder
For the french language for example :
Lign 36 in notepad++ :
Change
var FORCED_LOCALE = null;
To
var FORCED_LOCALE = 'fr'; (don't forgot punctuation)

Anyway i offered my help to complete the translation of the game to french.